Method of cleaning exhaust gas

ABSTRACT

An exhaust gas containing nitrogen oxides and particulate matter is cleaned by using an exhaust gas cleaner including a heat-resistant, porous filter; a porous ceramic powder layer formed on the filter; and a catalyst supported by the ceramic powder layer, the catalyst consisting essentially of (a) at least one of alkali metal elements, (b) cobalt and/or manganese, (c) vanadium, and (d) at least one of rare earth elements, whereby the nitrogen oxides are reduced by the particulate matter in the exhaust gas serving as a reducing agent.

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust gas cleaner and a method ofcleaning an exhaust gas by using such an exhaust gas cleaner, and moreparticularly to an exhaust gas cleaner capable of efficiently removingnitrogen oxides and fine carbonaceous particles simultaneously fromexhaust gases of diesel engines, etc., and a method of cleaning anexhaust gas by using such an exhaust gas cleaner.

Recently, fine carbonaceous particles (hereinafter referred to simply as"particulate matter") and nitrogen oxides (hereinafter referred tosimply as "NOx") contained in exhaust gases of diesel engines, etc. arecausing environmental problems. In particular, the particulate matterhaving an average particle size of 0.1-1 μm is likely to float in theair and be inhaled by breathing. As a result of recent clinical tests,it is confirmed that the particulate matter contains carcinogenicsubstances.

As a method for removing the particulate matter, there are twocandidates:

One method comprises the steps of trapping the particulate matter inheat-resistant filters by filtrating exhaust gases, and burning thetrapped particulate matter by a burner, an electrical heater, etc. whena pressure drop increases due to the particulate matter accumulated, toregenerate filters. The heat resistant filters may be honeycomb-typeceramic filters, foam-type ceramic filters having three-dimensionalnetwork structures, steel wool, wire mesh, etc. The other methodcomprises the step of trapping and self-burning the particulate matterby the action of catalysts supported by the above filters.

In the former method, as the efficiency for removing the particulatematter increases, the pressure drop increases more quickly, meaning thatthe filters are required to be regenerated more frequently with a highreliability, leading to an economical disadvantage.

In contrast, the latter method is considered to be excellent as long asa proper catalyst exists, which is capable of maintaining a catalyticactivity under the conditions of the exhaust gases of diesel engineswith which the catalyst is brought into contact (gas composition, gastemperature, etc).

However, since a diesel oil is used as a fuel in diesel engines, exhaustgases contain a large amount of SO₂. The oxygen concentration in exhaustgases varies in a wide range of 2-20%, depending upon the operationconditions of diesel engines. Under these conditions, there has been noestablished method of well igniting and burning fine carbon particlesaccumulated without causing secondary pollution.

For instance, as catalysts for removing particulate matter from exhaustgases, which have been proposed so far, there are precious metalcatalysts and base metal catalysts. The precious metal catalysts aredurable and function efficiently to oxidize CO and unburned hydrocarbons(hereinafter referred to simply as "HC"), but it is likely to convertSO₂ existing in exhaust gases to SO₃, leading to secondary pollution.Besides, there are problems such that ignition activity of soot in theparticulate matter is lowered. On the other hand, the base metalcatalysts are effective for removing the particulate matter, but theirdurability is poor.

Most of the catalysts for exhaust gas cleaners, which have been proposedhitherto, mainly function to lower the ignition temperature ofparticulate matter, leaving unsolved the problems of removing NOx fromexhaust gases of diesel engines having a large oxygen concentrationand/or a considerably variable oxygen concentration.

Japanese Patent Laid-Open No. 3-47539 discloses an exhaust gas cleanercomprising a heat-resistant, porous filter, a first catalyst supportedby the filter in the inlet region, and a second catalyst supported bythe filter in the outlet region, the first catalyst consistingessentially of (a) one or more alkali metal elements, (b) one or moreelements in Groups IB, IIA, IIB, transition metal elements of thePeriodic Table and Sn, and (c) one or more rare earth elements, and thesecond catalyst consisting essentially of at least one platinum-groupelement. By this exhaust gas cleaner, NOx and particulate matter areremoved by the first catalyst in the inlet region by using mainlyparticulate matter and HC as reducing agents, and HC, CO, and othertoxic gases are removed by the second catalyst in the outlet region.However, according to their research, the catalytic activity of thiscatalyst is not satisfactory even when any one of the transition metalelements is selected.

OBJECT AND SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an exhaustgas cleaner capable of efficiently removing not only particulate matterbut also nitrogen oxides from exhaust gas having a considerably variableoxygen concentration, particularly from exhaust gas containing largeamounts of nitrogen oxides, particulate matter and unburnedhydrocarbons.

Another object of the present invention is to provide an exhaust gascleaner capable of efficiently removing particulate matter, nitrogenoxides, HC, CO, etc. from exhaust gas having a considerably variableoxygen concentration at a relatively low temperature.

A further object of the present invention is to provide a method ofcleaning an exhaust gas by using the above exhaust gas cleaner.

As a result of intense research in view of the above objects, theinventors have found that by using an exhaust gas cleaner comprising aheat-resistant, porous filter with a porous ceramic powder which carriesa catalyst consisting essentially of at least one alkali metal element,particular transition elements and at least one rare earth metalelement, the particulate matter and the nitrogen oxides can beefficiently removed simultaneously from the exhaust gas having aconsiderably variable oxygen concentration, and the cleaning-efficiencyof the exhaust gas cleaner can be kept high for a long period of time.The inventors also have found that when the exhaust gas contains largeamounts of unburned hydrocarbons, the amount of the catalyst shouldrather be limited in order to increase the efficiency of removing NOx ata lower regeneration temperature. The inventors further have found thatby combining a first catalyst consisting of an alkali metal, aparticular transition metal and a rare earth metal in the inlet regionof the filter and a second catalyst of a Pt-group metal in the outletregion of the filter, such toxic gases as NOx, HC, CO, etc. as well asparticulate matter can be efficiently removed for a long period of time.The present invention is based on these findings.

Thus, the first exhaust gas cleaner according to the present inventioncomprises a heat-resistant, porous filter, a porous ceramic powder layerformed on the filter, and a catalyst supported by the ceramic powderlayer, the catalyst consisting essentially of:

(a) at least one of alkali metal elements;

(b) cobalt and/or manganese;

(c) vanadium; and

(d) at least one of rare earth elements.

The first method of cleaning an exhaust gas containing nitrogen oxidesand particulate matter according to the present invention comprisesusing the above first exhaust gas cleaner, whereby the nitrogen oxidesare reduced by the particulate matter in the exhaust gas serving as areducing agent.

The second exhaust gas cleaner according to the present inventioncomprises a heat-resistant, porous filter, and a catalytic layer formedon the filter, the catalytic layer comprising a uniform mixture of aceramic powder carrier, and a catalyst consisting essentially of:

(a) at least one of alkali metal elements;

(b) cobalt and/or manganese;

(c) vanadium; and

(d) at least one of rare earth elements,

the catalyst being 1-20 parts by weight and the ceramic powder carrierbeing 1-10 parts by weight per 100 parts by weight of the filter.

The second method of cleaning an exhaust gas containing nitrogen oxidesand particulate matter according to the present invention comprisesusing the above second exhaust gas cleaner, whereby the nitrogen oxidesare reduced by the particulate matter in the exhaust gas serving as areducing agent.

The third exhaust gas cleaner according to the present inventioncomprises a heat-resistant, porous filter, a porous ceramic powder layerformed on the filter, and a catalyst supported by the ceramic powderlayer, the catalyst consisting essentially of:

(a) at least one of alkali metal elements;

(b) at least one element selected from the group consisting of copper,cobalt, manganese and vanadium; and

(c) at least one of rare earth elements,

the porous ceramic powder layer being 3-15 parts by weight per 100 partsby weight of the filter, and the catalyst being 2.0-7.0 weight % basedon the porous ceramic powder layer.

The third method of cleaning an exhaust gas containing nitrogen oxides,particulate matter and unburned hydrocarbons according to the presentinvention comprises using the third exhaust gas cleaner, whereby thenitrogen oxides are reduced by the particulate matter and the unburnedhydrocarbons in the exhaust gas serving as reducing agents.

The fourth exhaust gas cleaner according to the present inventioncomprises a heat-resistant, porous filter, a first catalyst supported bythe filter in the inlet region, and a second catalyst supported by thefilter in the outlet region, the first catalyst consisting essentiallyof:

(a) at least one of alkali metal elements;

(b) at least one of elements selected from the group consisting ofcopper, cobalt, manganese and molybdenum;

(c) vanadium; and

(d) at least one of rare earth elements;

and the second catalyst consisting essentially of at least oneplatinum-group element.

The fourth method of cleaning an exhaust gas containing nitrogen oxides,particulate matter, unburned hydrocarbons and CO according to thepresent invention comprises using the fourth exhaust gas cleaner,whereby the nitrogen oxides are reduced by particulate matter andunburned hydrocarbons existing as reducing agents in the exhaust gas,and the unburned hydrocarbons and CO remaining in the exhaust gasflowing into the outlet region are oxidized by the second catalyst.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be explained in detail below.

[1] First exhaust gas cleaner

[A] Heat-resistant, porous filter

Since the heat-resistant, porous filter according to the presentinvention is exposed to a high-temperature exhaust gas, it is requiredto have an excellent heat resistance, and particularly an excellentthermal shock resistance. It is also required to have a necessaryparticulate matter-capturing capacity while causing pressure drop onlywithin the permitted range. Such materials for the heat-resistant,porous filter include ceramics such as alumina, silica, titania,zirconia and their composites such as silica-alumina, alumina-zirconia,alumina-titania, silica-titania, silica-zirconia, titania-zirconia,mullite, cordierite, etc. The filter may be in the form of a honeycombfilter or a foam filter, which is already known.

The shape and the size of the filter may vary depending upon itspurpose. When the filter is a cylindrical, ceramic foam-type, itpreferably has a diameter of 30-400 mm and a length of 5-30 mm. Theceramic foam-type filter preferably has a porosity of 40-80% and anaverage pore diameter of about 200-400 μm. If necessary, a plurality offilters may be stacked or arranged in tandem.

[B] Porous ceramic powder layer

The catalyst is supported by the above heat-resistant, porous filter viaa more porous ceramic powder layer formed on the porous filter.

The porous ceramic powder layer may be made of a porous ceramic materialhaving a large surface area such as titania, alumina, zirconia, silica,titania-alumina, alumina-zirconia, alumina-silica, titania-silica,titania-zirconia, etc. Preferably, the porous ceramic powder layer ismade of titania (TiO₂).

The amount of the ceramic carrier powder may be 3-15 parts by weight per100 parts by weight of the filter. When the amount of ceramic carrierpowder is less than 3 parts by weight, the ceramic carrier cannotsupport a sufficient amount of the catalyst. On the other hand, when theamount of the ceramic powder exceeds 15 parts by weight, the pressuredrop in the exhaust gas cleaner becomes too high. The preferred amountof ceramic powder is 5-12 parts by weight.

The porous ceramic powder layer is formed on the filter by awash-coating method, a sol-gel method, etc.

In the wash-coating method, the filter is immersed in a slurry of theabove porous ceramic carrier material and dried so that a carrier powderlayer is formed on the filter.

The sol-gel method comprises hydrolyzing organic salts (for instance,alkoxides) of the ceramic carrier-constituting metals; applying theresulting sol to the filter; bringing the coated filter into contactwith water vapor, etc. to form a layer composed of colloidal particles;and drying and burning it to convert it to a carrier layer for thecatalyst. For instance, when catalytic metals are to be supported by atitania (TiO₂) carrier layer, a solution of Ti alkoxide (for instance,Ti(O-isoC₃ H₇)₄) in alcohol is mixed with an acid such as CH₃ COOH,HNO₃, HCl, etc. to prepare a coating solution, and the filter isimmersed in the coating solution. After removing the filter from thecoating solution, it is reacted with vapor or water to cause gelation.After drying and burning, a thin titania coating is formed on a poroussurface of the filter. In the sol-gel method, the acid serves as ahydrolysis catalyst in the course of gelation. However, alkalis may beadded in place of the acids to conduct the hydrolysis reaction.

Although the above explanation has been made with respect to the case ofusing titania as a ceramic carrier material, any other ceramics may besimilarly used to support the catalyst by the sol-gel method. Forinstance, in the case of supporting the catalytic components by alumina,the same methods as above may be used except for using alkoxides of Al.Other porous carriers may be used in the same manner as above.

[C] Catalyst

(1) Composition

The catalyst supported by the above filter via the ceramic powder layerconsists essentially of (a) at least one of alkali metal elements; (b)Co and/or Mn, (c) V; and (d) at least one of rare earth elements.

(i) Alkali metal elements

The alkali metal element (a) is preferably selected from Na, K and Cs.Particularly when Cs is used, any unburned hydrocarbons includingsaturated hydrocarbons such as propane, etc. can be reacted with theNOx, so that the NOx is efficiently removed from the exhaust gas. Thisis due to the fact that the presence of Cs serves to increase theselectivity of the reaction of the hydrocarbons with NOx, therebyreducing the reaction between the hydrocarbons and oxygen in the exhaustgas.

The amount of the component (a), measured as an alkali metal itself(active species), is 10-50 weight % based on the total weight of thecatalyst (on a metal basis). When the amount of the alkali metal elementis smaller than 10 weight % or larger than 50 weight %, the efficiencyof the catalyst to remove particulate matter and NOx simultaneously islow. The preferred amount of the alkali metal element is 15-30 weight %.

(ii) Transition metal elements

The transition metal elements consist of (b) Co and/or Mn, and (c) V.

The amount of the component (b), measured as a metal itself (activespecies), is 15-65 weight % based on the total weight of the catalyst(on a metal basis). When the amount of Co and/or Mn is smaller than 15weight %, the ability of the catalyst to ignite particulate matter islow. On the other hand, when the amount of Co and/or Mn is larger than65 weight %, the function of HC to reduce the amount of NOx is reduced.The preferred amount of Co and/or Mn is 20-50 weight %.

The amount of the component (c), measured as a metal itself (activespecies), is 15-65% based on the total weight of the catalyst (on ametal basis). When the amount of V is smaller than 15 weight %, thecatalyst is less resistant to sulfur. On the other hand, when the amountof V is larger than 65 weight %, the efficiency of the catalyst toremove particulate matter and NOx simultaneously is low. The preferredamount of V is 20-50 weight %.

The total amount of the components (b) and (c) is 30-80 weight %,preferably 40-80 weight %.

(iii) Rare earth elements

The rare earth elements (d) are preferably Ce, La, Nd or Sm, etc., andMisch metal which is a mixture of rare earth elements may be used as therare earth elements.

The amount of the component (d), measured as a metal itself (activespecies), is 10-50 weight % based on the total weight of the catalyst(on a metal basis). When the amount of the rare earth element is smallerthan 10 weight % or larger than 50 weight %, the efficiency of thecatalyst to remove particulate matter and NOx simultaneously is low. Thepreferred amount of the rare earth element is 15-30 weight %.

The total amount of (a)+(b)+(c)+(d) as metallic active components isgenerally 1-40 weight %, preferably 5-30 weight % based on the ceramicpowder layer.

(2) Carrying catalyst by ceramic powder layer

After forming a porous ceramic carrier layer on the filter by thewash-coating method or the sol-gel method, etc., the carrierlayer-coated filter is immersed in aqueous solutions of carbonates,nitrates, acetates, hydroxides, etc. of catalytic components, dried andthen burned to obtain the filter supporting the catalyst via the ceramiclayer. The catalytic metal salts may be of any type, as long as they aresoluble in water, including carbonates, nitrates, acetates, hydroxides,etc. With respect to V, a solution of NH₄ VO₃ and oxalic acid may beused. Alkali metals and V may simultaneously be applied by using asolution of alkali vanadate.

(3) Function of catalyst

By using the catalyst consisting essentially of the above components,the particulate matter in the exhaust gas can be ignited and burned at arelatively low temperature, and NOx can be efficiently removed with theparticulate matter serving as a reducing agent. Specifically, when theparticulate matter in the exhaust gas is brought into contact with theabove catalyst in the presence of oxygen, the ignition temperature ofthe particulate matter is lowered. As a result, the particulate matteris burned (oxidized) at a temperature lower than about 450° C. At thesame time, NOx is reduced to N₂ by the particulate matter serving as areducing agent, whereby the exhaust gas can efficiently be cleaned. Thereason why the reduction of the NOx can efficiently be carried out at arelatively low temperature is that the components (a), (b), (c) and (d)in the catalyst show synergistic effects. By using Co and/or Mn and V asthe components (b) and (c), the durability of the catalyst is so highthat the catalyst can stably remove NOx for a long period of time.

The exhaust gas of diesel engines, etc. usually contains about 100-500ppm of the unburned hydrocarbons (HC) and about 200-4000 ppm of the NOx.

Typical examples of the HC may be propane, propylene, ethylene,acetylene, etc. When the unburned HC is hydrocarbons having unsaturatedbonds such as propylene, acetylene, etc., previously proposed catalystscontaining Cu alone as a catalytic component are effective to someextent to reduce NOx by using the unburned HC as reducing agents (see,for instance, Japanese Patent Laid-Open No. 63-100919). However, whensaturated hydrocarbons such as propane, etc. are used as reducingagents, the reduction reaction of NOx does not efficiently take place.This is due to the fact the reactivity of carbon-carbon bonds is in thefollowing order: triple bonds (acetylene, etc.)>double bonds (ethylene,propylene, etc.)>single bonds (propane, etc.), and that when saturatedhydrocarbons having poor reactivity such as propane are used, thereduction reaction of the NOx does not take place sufficiently in thepresence of the conventional catalysts.

However, when the catalyst of the present invention is used, the NOx canbe efficiently removed even when saturated hydrocarbons such as propane,etc. are used as reducing agents. Accordingly, the removal of NOx isremarkably improved with the catalyst of the present invention ascompared to the conventional catalysts.

[D] Method of cleaning exhaust gas

With the first exhaust gas cleaner, the exhaust gas is cleaned. Sincethe HC in the exhaust gas is mainly composed of methane, ethylene andacetylene, etc., the reduction reaction temperature of the NOx is200°-500° C., preferably 250°-450° C. When the reaction temperature istoo high, the burning of the HC itself takes place, making itineffective as a reducing agent for the NOx.

When the amount of the HC serving as a reducing agent in the exhaust gasis too small, the HC such as propane, propylene, etc. may be introducedinto the exhaust gas in an amount necessary for the reduction of theNOx, before the exhaust gas enters into the exhaust gas cleaner.

[2] Second exhaust gas cleaner

[A] Heat-resistant, porous filter

The heat-resistant, porous filter in the second exhaust gas cleaner maybe the same as in the first exhaust gas cleaner.

[B] Catalytic layer

The catalytic layer is composed of a uniform mixture of a catalyst and aceramic powder carrier.

(1) Catalyst

The catalyst itself may be the same as in the first exhaust gas cleaner.The amount of the catalyst is 1-20 parts by weight per 100 parts byweight of the filter. When the amount of the catalyst is less than 1parts by weight, it is difficult to remove nitrogen oxides andparticulate matter simultaneously from the exhaust gas. On the otherhand, even if the amount of the catalyst exceeds 20 parts by weight, theability of the exhaust gas cleaner to remove nitrogen oxides andparticulate matter would not increase accordingly. Therefore, the upperlimit of the amount of the catalyst is set at 20 parts by weight. Thepreferred amount of the catalyst is 5-15 parts by weight.

(2) Ceramic powder carrier

The ceramic powder carrier itself may be the same as in the firstexhaust gas cleaner. The amount of the ceramic carrier powder is 1-10parts by weight per. 0 100 parts by weight of the filter. When theamount of the ceramic carrier powder is less than 1 parts by weight, theceramic carrier cannot support a sufficient amount of the catalyst. Onthe other hand, when the amount of the ceramic carrier powder exceeds 10parts by weight, the pressure drop in the exhaust gas cleaner becomestoo high. The preferred amount of ceramic carrier powder is 2-6 parts byweight.

(3) Formation of catalytic layer

The catalytic layer may be formed on the filter by a wash-coatingmethod, a sol-gel method, etc.

In the wash-coating method, the ceramic carrier powder such as titaniais impregnated with solutions of the catalyst components, and the filteris then immersed in a slurry of the ceramic carrier powder carrying thecatalyst.

In the sol-gel method, a coating solution is first prepared by mixing asolution of the ceramic carrier material with solutions of the catalystcomponents, and the filter is then immersed in the coating solution. Forinstance, a solution of Ti alkoxide in alcohol is mixed with an acidsuch as CH₃ COOH, HNO₃, HCl, etc. and aqueous solutions of catalyticcomponent metal salts to prepare the coating solution, and the ceramicfilter is immersed in the coating solution. After removing the ceramicfilter from the coating solution, it is reacted with vapor or water toprepare a thin layer of colloidal particles (sol) on the filter, whichis then converted to a gel. The gel is dried and then burned to providea catalyst-supporting ceramic carrier layer. In the sol-gel method, theacid serves as a hydrolysis catalyst in the course of gelation. However,alkalis may be added in place of the acids to conduct the hydrolysisreaction.

By using this sol-gel method, the catalyst can be extremely uniformlydispersed in the ceramic filter, leading to an increase in catalyticactivity.

Incidentally, any other ceramics than TiO₂ may be similarly used asceramic carrier materials to support the catalyst by the sol-gel method,etc.

In the second exhaust gas cleaner having the catalytic layer comprisinga uniform mixture of the catalyst and the ceramic powder carrier, arelatively large amount of the catalyst can be carried even when theamount of the ceramic carrier powder is small, thereby lowering apressure drop. This means that a large amount of the catalyst can beapplied to the filter, even in the case of using ceramic powder in anamount small enough to prevent a pressure drop.

[C] Method of cleaning exhaust gas

With the second exhaust gas cleaner, the exhaust gas is cleaned underthe same conditions as in the first exhaust gas cleaner.

[3] Third exhaust gas cleaner

[A] Heat-resistant, porous filter

The heat-resistant, porous filter in the third exhaust gas cleaner maybe the same as in the first exhaust gas cleaner.

[B] Porous ceramic powder layer

The catalyst is supported by the above heat-resistant, porous filter viaa more porous ceramic powder layer formed on the porous filter.

The porous ceramic powder layer may be the same as in the first exhaustgas cleaner. In order to increase the reactivity of NO_(x) with theparticulate and HC, the porous ceramic powder layer is preferablytitania, zirconia, alumina, titania-zirconia, alumina-zirconia, etc.

The amount of the ceramic carrier powder is 3-15 parts by weight,preferably 5-12 parts by weight per 100 parts by weight of the filter.

[C] Catalyst

(1) Composition

The catalyst supported by the above filter via the ceramic powder layerconsists essentially of (a) at least one of alkali metal elements (Na,K, Cs, etc.); (b) at least one of Cu, Co, Mn and V; and (c) at least oneof rare earth elements (Ce, La, Nd, Sm, etc.).

(i) Alkali metal elements

The amount of the component (a), measured as an alkali metal itself(active species), is 5-50 weight % based on the total weight of thecatalyst (on a metal basis). When the amount of the alkali metal elementis smaller than 5 weight % or larger than 50 weight %, the efficiency ofthe catalyst to remove particulate matter and NOx simultaneously is low.The preferred amount of the alkali metal element is 10-30 weight %.

(ii) Transition metal elements

It is preferable to use a combination of V and at least one of Cu, Coand Mn. By this combination, the catalytic activity stably lasts long.

The amount of the component (b), measured as a metal itself (activespecies), is 30-80 weight % based on the total weight of the catalyst(on a metal basis). When the amount of the component (b) is smaller than30 weight %, the ability of the catalyst to ignite particulate matter islow. On the other hand, when the amount of the component (b) is largerthan 80 weight %, the function of HC to reduce the amount of NOx isreduced. The preferred amount of the component (b) is 40-70 weight %.

In the case of a combination of V and at least one of Cu, Co and Mn, aweight ratio of at least one of Cu, Co and Mn to V is preferably about5/1-1/15. When the weight ratio is larger than 5/1, the resistance ofthe catalyst to sulfur is low. On the other hand, when the weight ratiois smaller than 1/15, the efficiency of the catalyst to removeparticulate matter and NOx simultaneously is low.

(iii) Rare earth elements

The amount of the component (c), measured as a metal itself (activespecies), is 5-50% based on the total weight of the catalyst (on a metalbasis). When the amount of the rare earth element is smaller than 5weight % or larger than 50 weight %, the efficiency of the catalyst toremove particulate matter and NOx simultaneously is low. The preferredamount of the rare earth element is 10-30 weight %.

The amount of the catalyst (total amount of (a)+(b)+(c)) as metallicactive components is determined as 2.0-7.0 weight % based on the ceramicpowder layer, judging from the fact that high burning efficiency ofparticulate matter is obtained in the case of large amounts of catalystcomponents, while high reactivity of HC with NOx is obtained in the caseof small amounts of catalyst components.

When the amount of the catalyst is smaller than 2.0 weight %, sufficientcatalytic activity cannot be obtained. On the other hand, when theamount of the catalyst is larger than 7.0 weight %, the efficiency ofthe catalyst to remove particulate matter and NOx simultaneously israther low. The preferred amount of the catalyst is 2.0-5.0 weight %

(2) Carrying catalyst by ceramic powder layer

The same procedures as in the first exhaust gas cleaner may be used.

(3) Function of catalyst

By using the catalyst consisting essentially of the above components,NOx can be efficiently removed with the HC and the particulate matterserving as reducing agents.

[D] Method of cleaning exhaust gas

With the third exhaust gas cleaner, the exhaust gas is cleaned. Sincethe HC in the exhaust gas is mainly composed of methane, ethylene andacetylene, etc., the reduction reaction temperature of the NOx is200°-500° C., preferably 250°-450° C. When the reaction temperature istoo high, the burning of the HC itself takes place, making itineffective as a reducing agent for the NOx.

When the amount of the HC serving as a reducing agent in the exhaust gasis too small, the HC such a propane, propylene, etc. may be introducedinto the exhaust gas in an amount necessary for the reduction of theNOx, before the exhaust gas enters into the exhaust gas cleaner.

[4] Fourth exhaust gas cleaner

[A] Heat-resistant, porous filter

The heat-resistant, porous filter in the fourth exhaust gas cleaner maybe the same as in the first exhaust gas cleaner.

A volume ratio of the inlet region to the outlet region in the filter ispreferably 1/4-8/1. Incidentally, the inlet region and the outlet regionmay be formed in a single filter, or a filter carrying the firstcatalyst and a filter carrying the second catalyst may be combined atthe above volume ratio.

[B] Porous ceramic powder layer

The catalyst may be supported by the above heat-resistant, porous filtervia a more porous ceramic powder layer formed on the porous filter.

The porous ceramic powder layer may be the same as in the first exhaustgas cleaner. In order to increase the reactivity of NOx with theparticulate and HC, the porous ceramic powder layer is preferablytitania, zirconia, alumina, titania-zirconia, alumina-zirconia, etc.

The amount of the ceramic carrier powder may be 5-20 parts by weight per100 parts by weight of the filter both in the inlet and outlet regions.

In order to reduce pressure drop, the porous ceramic powder layer isformed on the filter by a wash-coating method, a sol-gel method, etc.The wash-coating method and the sol-gel method themselves may be thesame as in the first exhaust gas cleaner.

[C] Catalyst

(1) First catalyst

The catalyst supported by the inlet region of the filter consistsessentially of (a) at least one of alkali metal elements; (b) at leastone of Cu, Co Mn and Mo, (c) V; and (d) at least one of rare earthelements.

(i) Alkali metal elements

The amount of the component (a), measured as an alkali metal itself(active species), is 10-50 weight % based on the total weight of thecatalyst (on a metal basis). The preferred amount of the alkali metalelement is 15-30 weight %.

(ii) Transition metal elements

The amount of the component (b), measured as a metal itself (activespecies), is 15-65 weight % based on the total weight of the catalyst(on a metal basis). The preferred amount of the component (b) is 20-50weight %.

The amount of the component (c), measured as a metal itself (activespecies), is 15-65% based on the total weight of the catalyst (on ametal basis). The preferred amount of V is 20-50 weight %.

(iii) Rare earth elements

The rare earth elements (d) are preferably Ce, La, Nd or Sm, etc., andMisch metal which is a mixture of rare earth elements may be used as therare earth elements.

The amount of the component (d), measured as a metal itself (activespecies), is 10-50 weight % based on the total weight of the catalyst(on a metal basis). The preferred amount of the rare earth element is15-30 weight %.

The total amount of (a)+(b)+(c)+(d) as metallic active components ispreferably 0.05-6 weight % based on the ceramic powder layer.

(2) Second catalyst

The second catalyst used together with the first catalyst is composed ofa Pt-group metal having a high oxidation capability. The Pt-group metalis preferably Pt, Pd a mixture of Pt and Pd, or a mixture of Pt, Pd andRh. The second catalyst may contain Au and/or Ag to increase its abilityto clean the exhaust gas.

The second catalyst is disposed in the outlet region of the filter toremove the HC and CO remaining in the exhaust gas which has passedthrough the first catalyst region and entered into the second catalystregion.

The amount of the second catalyst is 0.1-1 weight % based on the ceramicpowder layer. When Au and/or Ag is contained, the amount of Au and/or Agis 50 parts by weight or less, preferably 10-20 parts by weight, per 100parts by weight of the Pt-group metal.

(3) Carrying catalyst by ceramic powder layer

The first and second catalysts may be applied to a single filter.Alternatively, they may be applied to separate filters which are thencombined such that the first catalyst is positioned in the inlet regionand the second catalyst is positioned in the outlet region.

In the latter case, the first catalyst may be applied to the filter inthe same manner as described above. The second catalyst may be appliedto the filter by immersing the filter in an aqueous solution ofchloride, etc. of the Pt-group metal and if necessary, an aqueoussolution of chloride, etc. of Au and/or Ag, and then drying and burning.

Further, by conducting light irradiation to the filter impregnated withan aqueous solution of platinum-group element compounds such aschlorides of Pt, Pd, Rh, etc., catalyst-carrying efficiency is extremelyincreased. The irradiation can be conducted by means of a mercury vaporlamp having a power output of 500 W or so. Alternatively, it is possibleto deposit the platinum-group element catalyst in the carrier powder bylight irradiation and then to coat the catalyst-supporting carrierpowder on the filter. By this light irradiation method, the catalyst canbe supported by the carrier powder in high dispersion, and the coatinglayer of the filter can be made thin, whereby pressure drop can beminimized in the high-density, thin-layer portion.

(4) Function of catalyst

By using the first and second catalysts, NOx can be efficiently removedwith the particulate matter and HC serving as reducing agents.Specifically, when the particulate matter in the exhaust gas is broughtinto contact with the first catalyst region in the presence of oxygen,the ignition temperature of the particulate matter is lowered. As aresult, the particulate matter is burned (oxidized) at a temperaturelower than about 400° C. At the same time, NOx is reduced to N₂ by theparticulate matter and HC serving as reducing agents, whereby theexhaust gas can efficiently be cleaned. By using a combination of atleast one of Cu, Co, Mn and Mo and V, the durability of the catalyst isso high that the catalyst can stably remove NOx for a long period oftime. Incidentally, in the first catalyst region, the generation of SO₃can be suppressed because the the first catalyst is composed of basemetal compounds.

From the exhaust gas which enters into the second catalyst region, theremaining HC and CO, etc. are removed by the second catalyst.

[D] Method of cleaning exhaust gas

Since the HC in the exhaust gas is mainly composed of methane, ethyleneand acetylene, etc., the reduction reaction temperature of the NOx is200°-500° C., preferably 250°-450° C. When the reaction temperature istoo high, the burning of the HC itself takes place, making itineffective as a reducing agent for the NOx.

When the amount of the HC serving as a reducing agent in the exhaust gasis too small, the HC such as propane, propylene, etc. may be introducedinto the exhaust gas in an amount necessary for the reduction of theNOx, before the exhaust gas enters into the exhaust gas cleaner.

Incidentally, in the above explanation, catalyst components areexpressed as metal elements. However, the base metal catalysts usuallyexist in the form of oxides. K, for instance, is actually in the form ofK₂ O. Accordingly, please note that the base metal catalysts may be inthe form of oxides.

It is also to be noted that the detailed description of the second tofourth exhaust gas cleaners are omitted for simplicity as long as theyare already given with respect to the first exhaust gas cleaner.Accordingly, please refer to the corresponding sections in the part ofthe first exhaust gas cleaner for detailed explanations.

The present invention will be described in further detail by way of thefollowing Examples. In each Example and Comparative Example, catalyticcomponents are described simply by metal elements for simplicity, andthe amount of each catalyst component is expressed by a weight of ametal component in the catalyst component. For instance, the amount ofCs₂ O is expressed by a weight of Cs.

EXAMPLE 1

A ceramic foam-type filter made of cordierite (apparent volume: 2 l,density: 0.65 g/ml) was coated with TiO₂ powder in an amount of 10weight % based on the filter, by a wash-coating method.

The coated filter was impregnated with 2.5 weight % of Co by using anaqueous solution of CoCl₂, 2.5 weight % of La by using an aqueoussolution of La(NO₃)₃, and 2.5 weight % of Cs by using an aqueoussolution of CsNO₃, each based on TiO₂. After drying the impregnatedfilter, it was burned at 700° C. Next, the burned filter was impregnatedwith 2.5 weight % of V by using an aqueous solution of NH₄ VO₃ andoxalic acid. After drying the impregnated filter, it was burned again at700° C. for 3 hours to produce an exhaust gas cleaner having thefollowing composition:

    Cs/Co/V/La (TiO.sub.2)

With respect to this exhaust gas cleaner, a regeneration temperature ofthe filter (expressed by a temperature at which pressure drop started todecrease by burning particulate matter) and a conversion rate of NOx toN₂ at the regeneration temperature were measured by using a dieselengine having a displacement of 510 cc. The regeneration temperature andthe conversion rate of NOx to N₂ were evaluated at two points, i.e., ata time when decrease in pressure drop was observed for the first time inan initial stage of the operation, and at a time when decrease inpressure drop was observed after the lapse of 10 hours from theinitiation of the operation. The diesel engine was operated at 1500 rpmunder a load of 90%. Under these conditions, the exhaust gas comprised90 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppm of NOx, 10%of O₂ and 200 ppm of SO₂. The results are shown in Table 1.

EXAMPLE 2

In the same manner as in Example 1, a ceramic foam-type filter made ofcordierite (apparent volume: 2 l, density: 0.65 g/ml) was coated withTiO₂ in an amount of 10 weight % based on the filter. The coated filterwas impregnated with 2.5 weight % of Co, 2.5 weight % of Ce, and 2.5weight % of Cs, each based on the TiO₂ carrier layer, by using aqueoussolutions of CoCl₂, Ce(NO₃)₃ and CsNO₃, respectively. The impregnatedfilter was dried and burned in the same manner as in Example 1. Next,the burned filter was impregnated with 2.5 weight % of V in the samemanner as in Example 1 to produce an exhaust gas cleaner having thefollowing composition:

    Cs/Co/V/Ce (TiO.sub.2)

EXAMPLE 3

In the same manner as in Example 2, an exhaust gas cleaner supporting2.5 weight % of Co, 2.5 weight % of La, 2.5 weight % of K and 2.5 weight% of V, each based on TiO₂ carrier layer, was produced by using aqueoussolutions of CoCl₂, La(NO₃)₃, KCl and NH₄ VO₃, respectively. Theresulting exhaust gas cleaner had the following composition:

    K/Co/V/La (TiO.sub.2)

EXAMPLE 4

In the same manner as in Example 2, an exhaust gas cleaner supporting2.5 weight % of Co, 2.5 weight % of Ce, 2.5 weight % of K and 2.5 weight% of V, each based on the TiO₂ carrier layer, was produced by usingaqueous solutions of CoCl₂, Ce(NO₃)₃, KCl and NH₄ VO₃, respectively. Theresulting exhaust gas cleaner had the following composition:

    K/Co/V/Ce (TiO.sub.2)

EXAMPLES 5 AND 6

In the same manner as in Example 1, catalyst components were applied tothe filter except for using manganese acetate in place of CoCl₂.

    Cs/Mn/V/La (TiO.sub.2): (Example 5)

    Cs/Mn/V/Ce (TiO.sub.2): (Example 6)

EXAMPLES 7 AND 8

In the same manner as in Example 2, catalyst components were applied tothe filter except for using manganese acetate in place of CoCl₂.

    K/Mn/V/La (TiO.sub.2): (Example 7)

    K/Mn/V/Ce (TiO.sub.2): (Example 8)

With respect to the resulting exhaust gas cleaners of Examples 2-8, aregeneration temperature and a conversion rate of NOx to N₂ at theregeneration temperature were measured in the same manner as in Example2. The results are shown in Table 1.

COMPARATIVE EXAMPLES 1-8

In the same manner as in Example 1, each of eight ceramic foam-typefilters made of cordierite (apparent volume: 2 l, density: 0.65 g/ml)was coated with TiO₂ in an amount of 10 weight % based on the filter, byusing a wash-coating method.

Each coated filter was immersed in such a combination of aqueoussolutions of La(NO₃)₃, Ce(NO₃)₃, CoCl₂, manganese acetate, CsNO₃ and KClas to produce an exhaust gas cleaner supporting the following catalyticmetals each in an amount of 2.5 weight % on a metal basis, dried andburned in the same manner as in Example 1.

    Cs/Co/La (TiO.sub.2): (Comparative Example 1)

    Cs/Co/Ce (TiO.sub.2): (Comparative Example 2)

    K/Co/La (TiO.sub.2): (Comparative Example 3)

    K/Co/Ce (TiO.sub.2): (Comparative Example 4)

    Cs/Mn/La (TiO.sub.2): (Comparative Example 5)

    Cs/Mn/Ce (TiO.sub.2): (Comparative Example 6)

    K/Mn/La (TiO.sub.2): (Comparative Example 7)

    K/Mn/Ce (TiO.sub.2): (Comparative Example 8)

With respect to each of these exhaust gas cleaners, a regenerationtemperature and a conversion rate of NOx to N₂ at the regenerationtemperature were measured in the same manner as in Example 1. Theresults are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                               Initial Stage.sup.(1)                                                                       After 10 Hours.sup.(2)                                            Regener- Conversion Regener-                                                                             Conversion                                         ation    Rate of    ation  Rate of                                            Temp.    NOx to N.sub.2.sup.(3)                                                                   Temp.  NOx to N.sub.2.sup.(3)                    No.      (°C.)                                                                           (%)        (°C.)                                                                         (%)                                       ______________________________________                                        Example 1                                                                              390      15         390    18                                        Example 2                                                                              395      12         395    15                                        Example 3                                                                              410      10         410    14                                        Example 4                                                                              410      10         410    14                                        Example 5                                                                              410      14         410    18                                        Example 6                                                                              415      12         415    16                                        Example 7                                                                              420      10         420    14                                        Example 8                                                                              420      10         420    14                                        Comparative                                                                            340      25         420    12                                        Example 1                                                                     Comparative                                                                            360      20         430    12                                        Example 2                                                                     Comparative                                                                            370      18         440    10                                        Example 3                                                                     Comparative                                                                            370      18         450    10                                        Example 4                                                                     Comparative                                                                            370      24         420    12                                        Example 5                                                                     Comparative                                                                            375      20         430    12                                        Example 6                                                                     Comparative                                                                            380      18         440    10                                        Example 7                                                                     Comparative                                                                            380      18         450    10                                        Example 8                                                                     ______________________________________                                         Note                                                                          .sup.(1) Measured at a time when decrease in pressure drop was first          appreciated.                                                                  .sup.(2) Measured at a time when decrease in pressure drop was first          appreciated after 10 hours from the initiation of passing exhaust gas         through the filter.                                                           .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before         entering into the filter, and the amount of (Xb) of NOx in exhaust gas        after passage through the filter, by using the formula: (Xa - Xb)/Xa.    

As is clear from Table 1, the exhaust gas cleaners of Examples 1-8 showhigh conversion rates of NOx to N₂ than those of Comparative Examples1-8, when 10 hours passed after the operation had started. Also, at atime after the lapse of 10 hours, the regeneration temperature of thefilter at which particulate matter is ignited and burned is lower inExamples than in Comparative Examples, meaning that exhaust gas isefficiently cleaned for a long period of time in the present invention.

EXAMPLE 9

To prepare a coating solution, a solution of Ti alkoxide (Ti(O-isoC₃H₇)₄) in alcohol was mixed with acetic acid, and then mixed with anaqueous solution of CoCl₂, an aqueous solution of La(NO₃)₃, an aqueoussolution of CsNO₃, and a mixture of an aqueous solution of NH₄ VO₃ andoxalic acid.

A ceramic foam-type filter made of cordierite (apparent volume: 0.25 l,density: 0.65 g/ml) was immersed in the resulting coating solution.After removing the filter from the coating solution, the impregnatedfilter was brought into contact with water vapor to cause gelation.After drying, it was burned again at 600° C. for 3 hours to produce afilter element provided with a catalytic layer. 4 filter elements werestacked to produce an exhaust gas cleaner having an apparent volume of 1l.

The amount of TiO₂ was 3 weight % based on the filter, and the amountsof Co, La, Cs and V on a metal basis were all 1.5 weight % based on thefilter. The composition of the exhaust gas cleaner was as follows:

    Cs/Co/V/La (TiO.sub.2)

With respect to this exhaust gas cleaner, a pressure drop due to thecapturing of particulate matter, a regeneration temperature of thefilter (expressed by a temperature at which pressure drop started todecrease by burning particulate matter) and a conversion rate of NOx toN₂ at the regeneration temperature were measured by using a dieselengine having a displacement of 500 cc. The regeneration temperature andthe conversion rate of NOx to N₂ were evaluated at two points, i.e., ata time when decrease in pressure drop was observed for the first time inan initial stage of the operation, and at a time when decrease inpressure drop was observed after the lapse of 10 hours from theinitiation of the operation. The diesel engine was operated at 2500 rpmunder a load of 80%. Under these conditions, the exhaust gas comprised90 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppm of NOx, 10%of O₂ and 200 ppm of SO₂. The results are shown in Table 2.

EXAMPLE 10

Example 9 was repeated except for using manganese acetate in place ofCoCl₂. The composition of the exhaust gas cleaner was as follows:

    Cs/Mn/V/La (TiO.sub.2)

EXAMPLE 11

Example 9 was repeated except for using Ce(NO₃)₃ in place of La(NO₃)₃,and changing the amount of ceramic carrier component. The composition ofthe exhaust gas cleaner was as follows:

    Cs/Co/V/Ce (TiO.sub.2)

The amount of TiO₂ was 4 weight % based on the filter, and the amountsof Co, Ce, Cs and V on a metal basis were all 1.5 weight % based on thefilter.

EXAMPLE 12

Example 10 was repeated except for using Ce(NO₃)₃ in place of La(NO₃)₃,and changing the amount of ceramic carrier component. The composition ofthe exhaust gas cleaner was as follows:

    Cs/Mn/V/Ce (TiO.sub.2)

The amount of TiO₂ was 4 weight % based on the filter, and the amountsof Mn, Ce, Cs and V on a metal basis were all 1.5 weight % based on thefilter.

EXAMPLE 13

A slurry of potassium titanate, La(NO₃)₃, Co(NO₃)₂, and a mixture of anaqueous solution of NH₄ VO₃ and oxalic acid was prepared.

The same filter as in Example 9 was immersed in the resulting slurry.After removing the filter from the slurry and drying, it was burned at700° C. for 3 hours to produce a filter element provided with acatalytic layer. 4 filter elements were stacked to produce an exhaustgas cleaner having an apparent volume of 1 l.

The amount of TiO₂ was 3 weight % based on the filter, the amount of Kon a metal basis was 3 weight % based on the filter, and the amounts ofCo, La and V on a metal basis were all 2 weight % based on the filter.The composition of the exhaust gas cleaner was as follows:

    K/Co/V/La (TiO.sub.2)

EXAMPLE 14

Example 13 was repeated except for using manganese acetate in place ofCo(NO₃)₂. The composition of the exhaust gas cleaner was as follows:

    K/Mn/V/La (TiO.sub.2)

EXAMPLE 15

Example 13 was repeated except for using Ce(NO₃)₃ in place of La(NO₃)₃.The composition of the exhaust gas cleaner was as follows:

    K/Co/V/Ce (TiO.sub.2)

EXAMPLE 16

Example 14 was repeated except for using Ce(NO₃)₃ in place of La(NO₃)₃.The composition of the exhaust gas cleaner was as follows:

    K/Mn/V/Ce (TiO.sub.2)

With respect to the resulting exhaust gas cleaners of Examples 10-16,the pressure drop, the regeneration temperature and the conversion rateof NOx to N₂ at the regeneration temperature were measured in the samemanner as in Example 9. The results are shown in Table 2.

COMPARATIVE EXAMPLES 9-16

In the same manner as in Example 9, each of eight ceramic foam-typefilters made of cordierite was formed with a catalytic layer of TiO₂containing catalyst components described below.

The amount of the catalytic layer of TiO₂ +catalyst components was 10weight % based on the filter, and the amount of each metal component ona metal basis was 2.5 weight % based on the filter.

    Cs/Co/La (TiO.sub.2): (Comparative Example 9)

    Cs/Mn/La (TiO.sub.2): (Comparative Example 10)

    Cs/Co/Ce (TiO.sub.2): (Comparative Example 11)

    Cs/Mn/Ce (TiO.sub.2): (Comparative Example 12)

    K/Co/La (TiO.sub.2): (Comparative Example 13)

    K/Mn/La (TiO.sub.2): (Comparative Example 14)

    K/Co/Ce (TiO.sub.2): (Comparative Example 15)

    K/Mn/Ce (TiO.sub.2): (Comparative Example 16)

With respect to each of the resulting exhaust gas cleaners ofComparative Examples 9-16, the pressure drop, the regenerationtemperature and the conversion rate of NOx to N₂ at the regenerationtemperature were measured in the same manner as in Example 9. Theresults are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                  Pressure drop                                                                            Regeneration                                                                             Conversion Rate                               No..sup.(2)                                                                             (mm Hg)    Temp(°C.)                                                                         of NOx to N.sub.2.sup.(3)                     ______________________________________                                                       Initial Stage.sup.(1)                                          Ex. 9     50         390        18                                            Ex. 10    52         400        17                                            Ex. 11    70         400        13                                            Ex. 12    70         405        14                                            Ex. 13    68         405        12                                            Ex. 14    67         405        12                                            Ex. 15    70         410        12                                            Ex. 16    71         410        10                                            Com. Ex. 9                                                                              95         390        18                                            Com. Ex. 10                                                                             96         395        16                                            Com. Ex. 11                                                                             100        410        15                                            Com. Ex. 12                                                                             102        412        14                                            Com. Ex. 13                                                                             100        415        13                                            Com. Ex. 14                                                                             98         415        12                                            Com. Ex. 15                                                                             95         415        13                                            Com. Ex. 16                                                                             96         415        13                                                           After 10 Hours.sup.(4)                                         Ex. 9                390        18                                            Ex. 10               390        17                                            Ex. 11               400        13                                            Ex. 12               405        14                                            Ex. 13               405        12                                            Ex. 14               405        12                                            Ex. 15               410        12                                            Ex. 16               410        10                                            Com. Ex. 9           430        15                                            Com. Ex. 10          430        12                                            Com. Ex. 11          435        12                                            Com. Ex. 12          435        10                                            Com. Ex. 13          440        10                                            Com. Ex. 14          445        10                                            Com. Ex. 15          445        10                                            Com. Ex. 16          445        10                                            ______________________________________                                         Note                                                                          .sup.(1) Measured at a time when decrease in pressure drop was first          appreciated.                                                                  .sup.(2) Ex.: Example, and Com. Ex.: Comparative Example.                     .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before         entering into the filter, and the amount of (Xb) of NOx in exhaust gas        after passage through the filter, by using the formula: (Xa - Xb)/Xa.         .sup.(4) Measured at a time when decrease in pressure drop was first          appreciated after 10 hours from the initiation of passing exhaust gas         through the filter.                                                      

As is clear from Table 2, the exhaust gas cleaners of Examples 9-16 showsmaller pressure drop than those of Comparative Examples 9-16, and evenafter the lapse of 10 hours, they show high conversion rates of NOx toN₂ than those of Comparative Examples 9-16. Also, at that time, theregeneration temperature of the filter at which particulate matter isignited and burned is lower in Examples than in Comparative Examples,meaning that exhaust gas is efficiently cleaned for a long period oftime in the present invention.

EXAMPLE 17

A ceramic foam-type filter made of cordierite (apparent volume: 2 l,density: 0.65 g/ml) was coated with TiO₂ powder in an amount of 10weight % based on the filter, by a wash-coating method.

The coated filter was impregnated with 1 weight % of Cu by using anaqueous solution of CuCl₂, 1 weight % of La by using an aqueous solutionof La(NO₃)₃, and 1 weight % of Cs by using an aqueous solution of CsNO₃,each based on TiO₂. After drying the impregnated filter, it was burnedat 700° C. In the resulting exhaust gas cleaner, metal components of Cu,La and Cs existed as metal oxides. The composition of the exhaust gascleaner was as follows:

    Cs/Cu/La (TiO.sub.2)

With respect to this exhaust gas cleaner, a regeneration temperature ofthe filter (expressed by a temperature at which pressure drop started todecrease by burning particulate matter) and a conversion rate of NOx toN₂ at the regeneration temperature were measured by using a dieselengine having a displacement of 510 cc. The diesel engine was operatedat 1500 rpm under a load of 90%. Under these conditions, the exhaust gascomprised 150 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppmof NOx, 10% of O₂ and 200 ppm of SO₂. The results are shown in Table 3.

EXAMPLE 18

In the same manner as in Example 17, a ceramic foam-type filter made ofcordierite was coated with TiO₂ in an amount of 10 weight % based on thefilter. The coated filter was impregnated with 1 weight % of Cu, 1weight % of Ce, and 1 weight % of Cs, each based on the TiO₂ carrierlayer, by using aqueous solutions of CuCl₂, Ce(NO₃)₃ and CsNO₃,respectively. The impregnated filter was dried and burned in the samemanner as in Example 17 to produce an exhaust gas cleaner having thefollowing composition:

    Cs/Cu/Ce (TiO.sub.2)

EXAMPLE 19

In the same manner as in Example 18, an exhaust gas cleaner supporting 1weight % of Cu, 1 weight % of La, and 1 weight % of K, each based onTiO₂ carrier layer, was produced by using aqueous solutions of CuCl₂,La(NO₃)₃, and KCl, respectively. The composition of the exhaust gascleaner was as follows:

    K/Cu/La (TiO.sub.2)

EXAMPLE 20

In the same manner as in Example 18, an exhaust gas cleaner supporting 1weight % of Cu, 1 weight % of Ce, and 1 weight % of K, each based on theTiO₂ carrier layer, was produced by using aqueous solutions of CuCl₂,Ce(NO₃)₃, and KCl, respectively. The composition of the exhaust gascleaner was as follows:

    K/Cu/Ce (TiO.sub.2)

EXAMPLES 21-24

Examples 17-20 were repeated except for using CoCl₂ in place of CuCl₂,and changing the types of ceramic carrier powder.

    Cs/Co/La (Al.sub.2 O.sub.3): (Example 21)

    Cs/Co/Ce (Al.sub.2 O.sub.3 -ZrO.sub.2): (Example 22)

    K/Co/La (ZrO.sub.2): (Example 23)

    K/Co/Ce (TiO.sub.2 -ZrO.sub.2): (Example 24)

EXAMPLES 25-28

Examples 17-20 were repeated except for using manganese acetate in placeof CuCl₂.

    Cs/Mn/La (TiO.sub.2): (Example 25)

    Cs/Mn/Ce (TiO.sub.2): (Example 26)

    K/Mn/La (TiO.sub.2): (Example 27)

    K/Mn/Ce (TiO.sub.2): (Example 28)

With respect to the resulting exhaust gas cleaners of Examples 18-28, aregeneration temperature and a conversion rate of NOx to N₂ at theregeneration temperature were measured in the same manner as in Example17. The results are shown in Table 3.

COMPARATIVE EXAMPLE 17

In the same manner as in Example 17, a ceramic foam-type filter made ofcordierite was coated with TiO₂ in an amount of 10 weight % based on thefilter, by using a wash-coating method. The coated filter wasimpregnated with Cu in an amount of 1 weight % based on the filter byusing an aqueous solution of CuCl₂, dried and burned in the same manneras in Example 17.

With respect to the resulting exhaust gas cleaner, a regenerationtemperature and a conversion rate of NOx to N₂ at the regenerationtemperature were measured in the same manner as in Example 17. Theresults are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                     Regeneration                                                                             Conversion Rate of                                    No.          Temp. (°C.)                                                                       NOx to N.sub.2.sup.(1) (%)                            ______________________________________                                        Example 17   340        50                                                    Example 18   360        40                                                    Example 19   370        35                                                    Example 20   370        35                                                    Example 21   360        45                                                    Example 22   365        35                                                    Example 23   380        30                                                    Example 24   380        30                                                    Example 25   385        40                                                    Example 26   390        35                                                    Example 27   395        30                                                    Example 28   395        30                                                    Comparative  450        20                                                    Example 17                                                                    ______________________________________                                         Note                                                                          .sup.(1) Calculated from the amount (Xa) of NOx in exhaust gas before         entering into the filter, and the amount of (Xb) of NOx in exhaust gas        after passage through the filter, by using the formula: (Xa - Xb)/Xa.    

As is clear from Table 3, the exhaust gas cleaners of Examples 17-28show as high conversion rates of NOx to N₂ as 30% or more. Also, at theregeneration temperature of the filter at which particulate matter isignited and burned, exhaust gas is efficiently cleaned in the presentinvention.

EXAMPLE 29

A ceramic foam-type filter made of cordierite (apparent volume: 2 l,density: 0.65 g/ml) was coated with TiO₂ powder in an amount of 10weight % based on the filter, by a wash-coating method.

The coated filter was impregnated with 1 weight % of Cu by using anaqueous solution of CuCl₂, 1 weight % of La by using an aqueous solutionof La(NO₃)₃, and 1 weight % of Cs by using an aqueous solution of CsNO₃,each based on TiO₂. After drying the impregnated filter, it was burnedat 700° C. Next, the burned filter was impregnated with 1 weight % of Vby using an aqueous solution of NH₄ VO₃ and oxalic acid. After dryingthe impregnated filter, it was burned again at 700° C. for 3 hours toproduce an exhaust gas cleaner.

    Composition: Cs/Cu/La/V (TiO.sub.2)

With respect to this exhaust gas cleaner, a regeneration temperature ofthe filter and a conversion rate of NOx to N₂ at the regenerationtemperature were measured by using a diesel engine having a displacementof 510 cc. The regeneration temperature and the conversion rate of NOxto N₂ were evaluated at two points, i.e., at a time when decrease inpressure drop was observed for the first time in an initial stage of theoperation, and at a time when decrease in pressure drop was observedafter the lapse of 10 hours from the initiation of the operation. Thediesel engine was operated at 1500 rpm under a load of 90%. Under theseconditions, the exhaust gas comprised 150 ppm of HC (total ofhydrocarbons), 460 ppm of CO, 480 ppm of NOx, 10% of O₂ and 200 ppm ofSO₂. The results are shown in Table 4.

EXAMPLE 30

Example 29 was repeated except for using Al₂ O₃ in an amount of 10weight % based on the filter as a ceramic carrier powder, andimpregnating 1 weight %, based on the Al₂ O₃ layer, of Ce by usingCe(NO₃)₃ in place of La(NO₃)₃.

    Composition: Cs/Cu/Ce/V (Al.sub.2 O.sub.3)

EXAMPLE 31

Example 29 was repeated except for using Al₂ O₃ -ZrO₂ in an amount of 10weight % based on the filter as a ceramic carrier powder, andimpregnating 1 weight %, based on the Al₂ O₃ -ZrO₂ layer, of K by usingKCl in place of CsNO₃.

    Composition: K/Cu/La/V (Al.sub.2 O.sub.3 -ZrO.sub.2)

EXAMPLE 32

Example 30 was repeated except for using ZrO₂ in an amount of 10 weight% based on the filter as a ceramic carrier powder, and impregnating 1weight %, based on the ZrO₂ layer, of K by using KCl in place of CsNO₃.

    Composition: K/Cu/Ce/V (ZrO.sub.2)

EXAMPLES 33-36

Examples 29-32 were repeated except for using CoCl₂ in place of CuCl₂,and changing the types of ceramic carrier powder.

    Cs/Co/La/V (TiO.sub.2 -ZrO.sub.2): (Example 33)

    Cs/Co/Ce/V (TiO.sub.2): (Example 34)

    K/Co/La/V (TiO.sub.2): (Example 35)

    K/Co/Ce/V (TiO.sub.2): (Example 36)

EXAMPLES 37-40

Examples 29-32 were repeated except for using manganese acetate in placeof CuCl₂, and changing the types of ceramic carrier powder.

    Cs/Mn/La/V (TiO.sub.2): (Example 37)

    Cs/Mn/Ce/V (TiO.sub.2): (Example 38)

    K/Mn/La/V (TiO.sub.2): (Example 39)

    K/Mn/Ce/V (TiO.sub.2): (Example 40)

COMPARATIVE EXAMPLE 18

In the same manner as in Example 29, a ceramic foam-type filter made ofcordierite was coated with TiO₂ in an amount of 10 weight % based on thefilter, by using a wash-coating method. The coated filter wasimpregnated with Cu in an amount of 1 weight % based on the filter byusing an aqueous solution of CuCl₂, dried and burned in the same manneras in Example 29.

With respect to the resulting exhaust gas cleaners of Examples 30-40 andComparative Example 18, a regeneration temperature and a conversion rateof NOx to N₂ at the regeneration temperature were measured in the samemanner as in Example 29. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                               Initial Stage.sup.(1)                                                                       After 10 Hours.sup.(2)                                            Regener- Conversion Regener-                                                                             Conversion                                         ation    Rate of    ation  Rate of                                            Temp.    NOx to N.sub.2.sup.(3)                                                                   Temp.  NOx to N.sub.2.sup.(3)                    No.      (°C.)                                                                           (%)        (°C.)                                                                         (%)                                       ______________________________________                                        Example 29                                                                             380      40         382    38                                        Example 30                                                                             385      35         385    32                                        Example 31                                                                             400      25         403    22                                        Example 32                                                                             400      25         405    22                                        Example 33                                                                             385      30         390    28                                        Example 34                                                                             390      25         395    23                                        Example 35                                                                             405      20         408    19                                        Example 36                                                                             405      20         408    18                                        Example 37                                                                             400      30         410    28                                        Example 38                                                                             405      25         415    24                                        Example 39                                                                             410      20         420    18                                        Example 40                                                                             410      20         420    18                                        Comparative                                                                            450      20         500    10                                        Example 18                                                                    ______________________________________                                         Note                                                                          .sup.(1) Measured at a time when decrease in pressure drop was first          appreciated.                                                                  .sup.(2) Measured at a time when decrease in pressure drop was first          appreciated after 10 hours from the initiation of passing exhaust gas         through the filter.                                                           .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before         entering into the filter, and the amount of (Xb) of NOx in exhaust gas        after passage through the filter, by using the formula: (Xa - Xb)/Xa.    

As is clear from Table 4, the exhaust gas cleaners of Examples 29-40show high conversion rates of NOx to N₂ than those of ComparativeExample 18, when 10 hours passed after the operation had started. Also,at a time after the lapse of 10 hours, the regeneration temperature ofthe filter at which particulate matter is ignited and burned is lower inExamples than in Comparative Example, meaning that exhaust gas isefficiently cleaned for a long period of time in the present invention.

EXAMPLE 41

A ceramic foam-type filter made of cordierite (apparent volume: 1 l,density: 0.65 g/ml) was coated with TiO₂ powder in an amount of 10weight % based on the filter, by a wash-coating method. The coatedfilter was impregnated with 1 weight % of Cs by using an aqueoussolution of CsNO₃, 2 weight % of Cu by using an aqueous solution ofCuCl₂, 1 weight % of La by using an aqueous solution of La(NO₃)₃, eachbased on TiO₂. Next, the impregnated filter was further impregnated with1 weight % of V by using an aqueous solution of NH₄ VO₃ and oxalic acid.After drying the impregnated filter, it was burned at 700° C. for 3hours to prepare a first filter.

The same type of ceramic foam-type filter as above (apparent volume: 0.2l, density: 0.65 g/ml) was coated with γ-Al₂ O₃ powder in an amount of10 weight % based on the filter, by a wash-coating method. The coatedfilter was impregnated with 0.1 weight %, based on γ-Al₂ O₃, of Pt byusing an aqueous solution of H₂ PtCl₆. After drying the impregnatedfilter, it was burned at 700° C. to prepare a second filter.

The first and second filters were stacked such that the first filter waspositioned on the inlet side, while the second filter was positioned onthe outlet side, to produce an exhaust gas cleaner having the followingcomposition:

    Cs/Cu/V/La/TiO.sub.2 -Pt/Al.sub.2 O.sub.3

With respect to this exhaust gas cleaner, a regeneration temperature ofthe filter and a conversion rate of NOx to N₂ at the regenerationtemperature were measured by using a diesel engine having a displacementof 510 cc. The regeneration temperature and the conversion rate of NOxto N₂ were evaluated at two points, i.e., at a time when decrease inpressure drop was observed for the first time in an initial stage of theoperation, and at a time when decrease in pressure drop was observedafter the lapse of 10 hours from the initiation of the operation. Thediesel engine was operated at 2500 rpm under a load of 80%. Under theseconditions, the exhaust gas comprised 90 ppm of HC (total ofhydrocarbons), 460 ppm of CO, 480 ppm of NOx, 8% of O₂ and 100 ppm ofSO₂. The results are shown in Table 5.

EXAMPLE 42

In the same manner as in Example 41, a first filter was prepared bycoating TiO₂ in an amount of 10 weight % based on the filter andimpregnating it with 1 weight % of Cs by using an aqueous solution ofCsNO₃, 2 weight % of Cu by using an aqueous solution of CuCl₂, 1 weight% of Ce by using an aqueous solution of Ce(NO₃)₃, each based on TiO₂,and then impregnating the filter with 1 weight % of V.

This first filter was stacked with the second filter obtained in Example41 to produce an exhaust gas cleaner having the following composition:

    Cs/Cu/V/Ce/TiO.sub.2 -Pt/Al.sub.2 O.sub.3

EXAMPLE 43

In the same manner as in Example 41, a first filter was prepared bycoating TiO₂ in an amount of 10 weight % based on the filter andimpregnating it with 1 weight % of K by using an aqueous solution of K₂CO₃, 2 weight % of Cu by using an aqueous solution of Cu(NO₃)₂, and 1weight % of La by using an aqueous solution of La(NO₃)₃, each based onTiO₂, and then impregnating the filter with 1 weight % of V.

This first filter was stacked with the second filter obtained in Example41 to produce an exhaust gas cleaner having the following composition:

    K/Cu/V/La/TiO.sub.2 -Pt/Al.sub.2 O.sub.3

EXAMPLE 44

In the same manner as in Example 41, a first filter was prepared bycoating TiO₂ in an amount of 10 weight % based on the filter andimpregnating it with 1 weight % of K by using an aqueous solution of K₂CO₃, 2 weight % of Cu by using an aqueous solution of Cu(NO₃)₂, and 1weight % of Ce by using an aqueous solution of Ce(NO₃)₃, each based onTiO₂, and then impregnating the filter with 1 weight % of V.

This first filter was stacked with the second filter obtained in Example41 to produce an exhaust gas cleaner having the following composition:

    K/Cu/V/Ce/TiO.sub.2 -Pt/Al.sub.2 O.sub.3

With respect to the resulting exhaust gas cleaners of Examples 42-44, aregeneration temperature and a conversion rate of NOx to N₂ at theregeneration temperature were measured in the same manner as in Example41. The results are shown in Table 5.

EXAMPLES 45-47

The same first filters as in Examples 42-44 were prepared.

On the other hand, TiO₂ was dispersed in an aqueous solution of PdCl₂(Example 45), in an aqueous solution of H₂ PtCl₆ (Example 46), and in anaqueous solution of H₂ PtCl₆ and PdCl₂ (Example 47), to impregnate itwith 0.1 weight % of Pd (Example 45), 0.1 weight % of Pt (Example 46),and 0.1 weight % of Pt+0.1 weight % of Pd (Example 47). Each ceramicfoam-type filter made of cordierite (apparent volume: 0.2 l, density:0.65 g/ml) was coated with the above TiO₂ carrying the Pt-group metal toprepare a second filter.

The first and second filters were stacked in the same manner as inExample 41, to produce an exhaust gas cleaner having the followingcomposition:

    Cs/Cu/V/La/TiO.sub.2 -Pd/TiO.sub.2 (Example 45)

    Cs/Cu/V/Ce/TiO.sub.2 -Pt/TiO.sub.2 (Example 46)

    K/Cu/V/La/TiO.sub.2 -Pt, Pd/TiO.sub.2 (Example 47)

With respect to the resulting exhaust gas cleaners of Examples 45-47, aregeneration temperature and a conversion rate of NOx to N₂ at theregeneration temperature were measured in the same manner as in Example41. The results are shown in Table 5.

COMPARATIVE EXAMPLE 19

Example 41 was repeated except for using only Cu in the catalyst appliedto the first filter. The resulting exhaust gas cleaner had the followingcomposition:

    Cu/TiO.sub.2 -Pt/Al.sub.2 O.sub.3

A regeneration temperature and a conversion rate of NOx to N₂ at theregeneration temperature were measured in the same manner as in Example41. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                Regen.  Conversion Rate of NOx to N.sub.2.sup.(3)                     No..sup.(2)                                                                             Temp(°C.)                                                                        NOx        CO   HC                                        ______________________________________                                                      Initial Stage.sup.(1)                                           Ex. 41    380       10         55   40                                        Ex. 42    385       12         55   40                                        Ex. 43    400        8         57   39                                        Ex. 44    410        7         58   39                                        Ex. 45    385       10         50   30                                        Ex. 46    385       12         60   50                                        Ex. 47    405       15         70   60                                        Com. Ex. 19                                                                             450        5         30   20                                                      After 10 Hours.sup.(4)                                          Ex. 41    390        8         50   38                                        Ex. 42    395       10         50   38                                        Ex. 43    400        8         55   38                                        Ex. 44    405        7         55   38                                        Ex. 45    390        8         45   28                                        Ex. 46    390       10         58   48                                        Ex. 47    400       12         65   58                                        Com. Ex. 19                                                                             530        2         15   10                                        ______________________________________                                         Note                                                                          .sup.(1) Measured at a time when decrease in pressure drop was first          appreciated.                                                                  .sup.(2) Ex.: Example, and Com. Ex.: Comparative Example.                     .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before         entering into the filter, and the amount of (Xb) of NOx in exhaust gas        after passage through the filter, by using the formula: (Xa - Xb)/Xa.         .sup.(4) Measured at a time when decrease in pressure drop was first          appreciated after 10 hours from the initiation of passing exhaust gas         through the filter.                                                      

As is clear from Table 5, the exhaust gas cleaners of Examples 41-47show higher cleaning efficiency than those of Comparative Example 19even after the lapse of 10 hours. Also, at that time, the regenerationtemperature of the filter at which particulate matter is ignited andburned is lower in Examples than in Comparative Example.

As described above in detail, by using the exhaust gas cleaner accordingto the present invention, both particulate matter and NOx canefficiently be removed from the exhaust gas. Such exhaust gas cleanersare effective for cleaning exhaust gases even at a low temperature, suchas those of diesel engines. Also, since the catalyst contains Co and/orMn and V as transition metals, the efficiency of the catalyst is kepthigh for a long period of time even when it is in contact with anexhaust gas containing a high concentration of SO₂.

In the first exhaust gas cleaner of the present invention, since thecatalyst is supported by the porous ceramic carrier powder layer formedon the filter by a wash-coating method, a sol-gel method, etc., thecatalyst can be uniformly supported on the filter in a highconcentration, leading to a good catalytic activity.

In the second exhaust gas cleaner of the present invention, since auniform mixture of the catalyst with the porous ceramic carrier powderis supported by the filter, it suffers from only little pressure drop,thereby effecting the removal of both particulate matter and NOxefficiently.

In the third exhaust gas cleaner of the present invention, since thecatalyst having a particular composition is supported by the filter inan extremely small amount, high reactivity of HC with particulate matterand unburned hydrocarbons can be maintained for a long period of time.

In the fourth exhaust gas cleaner of the present invention, since thefirst and second catalysts are combined, NOx and particulate matter, HC,CO, etc. are efficiently removed from the exhaust gas for a long periodof time.

The exhaust cleaner and the exhaust gas-cleaning method according to thepresent invention are highly useful for cleaning the exhaust gas ofdiesel engines, etc.

What is claimed is:
 1. A method of cleaning an exhaust gas containingnitrogen oxides and particulate matter, which comprises using an exhaustgas cleaner comprising a heat-resistant porous filter; a porous ceramicpowder layer formed on said filter, in the amount of 3-15 parts byweight per 100 parts by weight of the filter; and a catalyst in theamount of 1-40 weight % based on the ceramic powder layer and supportedby said ceramic powder layer, said catalyst consisting essentially of(a) at least one alkali metal element in the amount of 10-50 weight %based on the total weight of the catalyst on a metal basis, (b) cobalt,manganese, or a combination of cobalt and manganese, in the amount of15-65 weight % based on the total weight of the catalyst on a metalbasis, (c) vanadium in the amount of 15-65 weight % based on the totalweight of the catalyst on a metal basis, with the total amount ofcomponents (b) and (c) being 30-80 weight %, and (d) at least one rareearth element in the mount of 10-50 weight % based on the total weightof the catalyst on a metal basis, whereby said nitrogen oxides arereduced by said particulate matter in said exhaust gas serving as areducing agent.
 2. A method of cleaning an exhaust gas containingnitrogen oxides and particulate matter, which comprises using an exhaustgas cleaner comprising a heat-resistant, porous filter; and a catalyticlayer formed on said filter, said catalytic layer comprising a uniformmixture of a ceramic powder carrier, and a catalyst consistingessentially of (a) at least one alkali metal element in the amount of10-50 weight % based on the total weight of the catalyst on a metalbasis, (b) cobalt, manganese, or a combination of cobalt and manganesein the amount of 15-65 weight % based on the total weight of thecatalyst on a metal basis, (c) vanadium in the amount of 15-65 weight %based on the total weight of the catalyst on a metal basis, with thetotal amount of components (b) and (c) being 30-80 weight %, and (d) atleast one rare earth element in the amount of 10-50 weight % based onthe total weight of the catalyst on a metal basis, said catalyst being1-20 parts by weight and said ceramic powder carrier being 1-10 parts byweight per 100 parts by weight of said filter, whereby said nitrogenoxides are reduced by said particulate matter in said exhaust gasserving as a reducing agent.
 3. A method of cleaning an exhaust gascontaining nitrogen oxides, particulate matter, unburned hydrocarbonsand CO, which comprises using an exhaust gas cleaner comprising aheat-resistant, porous filter, a first catalyst supported by said filterin the inlet region and a second catalyst supported by said filter inthe outlet region, said first catalyst consisting essentially of (a) atleast one alkali metal element in the amount of 10-50 weight % based onthe total weight of the catalyst on a metal basis, (b) at least oneelement selected from the group consisting of copper, cobalt, manganeseand molybdenum in the amount of 15-65 weight % based on the total weightof the catalyst on a metal basis, (c) vanadium in the amount of 15-65weight % based on the total weight of the catalyst on a metal basis, and(d) at least one rare earth element in the amount of 10-50 weight %based on the total weight of the catalyst on a metal basis, and saidsecond catalyst consisting essentially of at least one platinum-groupelement in the amount of 0.1-1 weight % based on the weight of theceramic layer, whereby said nitrogen oxides are reduced by saidparticulate matter and unburned hydrocarbons existing as reducing agentsin said exhaust gas, and the unburned hydrocarbons and CO remaining inthe exhaust gas flowing into said outlet region are oxidized by saidsecond catalyst.